Increasingly widespread utilization of forest resources has led to a scarcity of old-growth timbers in many parts of the world. Old-growth timber is particularly valuable because it generally contains a higher percentage of “mature” wood per unit volume. In contrast, timber from plantations or other environments in which trees are urged to reach a harvestable size as soon as possible generally has a higher percentage of “juvenile” wood per unit volume. Lumber having a greater percentage of mature wood tends to be harder than lumber having a greater percentage of juvenile wood. For example, FIG. 1 shows a comparison between the hardness of maple (an old-growth wood or a naturally hard wood) and alder based. In addition to old-growth timber, there are other types of timber that are naturally hard. Hardness is an important mechanical property for many applications including the manufacture of cabinets, flooring, furniture, decorative objects, and other products using wood as a material. Thus, old-growth timber or naturally hard timber is in high demand.
In addition to hardness, a number of other mechanical and aesthetic properties are desirable for the applications listed above. For example, a wood that is used for flooring material should be able to withstand exposure to water or humidity without significant swelling or shrinkage. The wood's ability to be stained or varnished (refinishability) is also important for many applications. Abrasion resistance, workability, and color modification are also desired characteristics.
Due to the shortage of old-growth timber and other naturally hard timber, the industry has developed numerous methods for treating available wood with inadequate properties to make it more closely resemble those of old-growth timber and naturally hard timber. Many of these treatment methods are concerned with increasing hardness. Altering the hardness of wood generally involves increasing the wood's density through either chemical or mechanical means.
Chemical methods for increasing wood density often involve impregnating the wood with polymers, resins, waxes, or other chemical treatments to fill voids in the structure. One such method is used widely to create a product known in the industry as “compreg” or compressed and impregnated wood. See Stamm, A. J., R. M. Seborg, 1941, Resin treated, laminated, compressed wood, Trans. Am. Inst. Chem. Eng., 37:385-397. The method involves treating solid wood or veneer with water-soluble phenol formaldehyde resin and compressing it to a desired specific gravity and thickness. One drawback of this method is that the chemicals used in this and other chemical hardness enhancing processes pose a number of health, safety, and environmental risks. Acquiring the equipment and facilities to perform such a procedure may also be more expensive than buying highly priced old-growth timber or naturally hardwood with adequate mechanical properties. In some cases, the chemicals themselves be a large portion of this cost. Additionally some resins used for impregnation has a dark brown color which affects the appearance of the final product as well as its ability to accept a varnish or stain.
Mechanical methods for increasing wood density generally involve redistributing lignin throughout the wood's structure. Wood is composed of essentially three components: cellulose, hemicellulose, and lignin. Lignin a group of phenolic polymers that confer strength and rigidity to the woody cell wall of plants. Thus, redistributing lignin throughout the wood can increase its overall strength.
U.S. Pat. No. 2,453,679 discloses a mechanical method for compressing a wood to cause lignin to flow within the structure. According to the disclosure, the method involves compressing a wood having a moisture content between 6% and 12% in a press set to an initial temperature between 210° F. and 240° F. The wood is compressed to a specific gravity of 1.3-1.4, and then the press is adjusted to a temperature between 330° F. and 360° F. The wood is held at this temperature for 5 to 30 minutes while pressure is maintained. Thereafter the wood is cooled under pressure to a temperature of 200° F. or lower before removal from the press. One drawback of this method is that if the wood is removed before it is completely cooled, it will tend to undergo “springback” or recovery from compression when exposed to moisture. In addition, density changes in the wood tend to not be uniform, leaving end pieces that must be trimmed away because they are lighter in color and more unstable than the rest of the wood.
U.S. Pat. No. 7,404,422 discloses another mechanical method for densifying wood components known as “viscoelastic thermal compression.” The method involves heating and conditioning wood to its glass transition temperature and subsequently compressing the wood. After compression, an annealing process is performed which involves holding the wood at a pressure between 2000 kPa and 4000 kPa and a temperature between 350° F. and 440° F. for about 60 to 120 seconds. After annealing the wood is cooled below the glass transition temperature. One drawback of this method is that heating to a temperature over 400° F. can actually cause the lignin to decompose. Another drawback is that although the process can increase density of the wood, the increase is not always uniform. Additionally the high heat can scorch the wood, thus adversely affecting its aesthetic appearance and coloring.
Thus, there is a need to develop a method for enhancing the hardness and dimensional stability of wood in a manner that preserves the color and appearance of the wood for applications where aesthetics are an important factor. There is also a need to enhance the hardness of wood by causing a uniform increase in density. It would also be an improvement to develop such a process that can be implemented using conventional equipment at minimal cost. There is also a need to develop a method for enhancing hardness that also has a desirable effect on other wood properties such as abrasion resistance, refinishability, workability, and color modification.